This invention relates generally to insulated gate field effect transistors (IGFETs) and particularly MOSFET transistor circuits in which hot carrier injection is experienced, and more particularly the invention relates to a current source or current mirror bias circuit in which a reference device has characteristics which track degradation in a main device due to hot carrier injection in the main device.
A conventional current source or current mirror circuit comprises a transistor having base-collector (bipolar) or gate-drain (FET) connected whereby the transistor functions as a diode. The circuit is typically used as a biasing element for other transistors so that common operating characteristics are shared by many transistor devices biased by a common current source. Current through the reference transistor is mirrored (exactly or fractionally) in the main transistor.
However, a main transistor with hot carrier injection in the channel/drain regions experiences a degradation of operating characteristics which is not experienced in the reference transistor due to the fact that the drain-gate electric field is zero. The present invention is directed to providing a modified current source bias circuit in which the reference transistor operating characteristics degrade in accordance with hot carrier injection degradation in the main transistor.